docs: def effective_entries(self):
          """
          Number of effective entries in this bin.
          The number of unweighted entries this bin would need to
          contain in order to have the same statistical power as this
          bin with possibly weighted entries, estimated by:
  
              (sum of weights) ** 2 / (sum of squares of weights)
  
          """
          sum_w2 = self.sum_w2
          if sum_w2 == 0:
              return abs(self.value)
          return (self.value ** 2) / sum_w2